[Leonatus]

It seems that I found the real culprit: The text transformed into epub ended always (also with the help of the online tool, what I only now realized) at the expression: "laterna magica". As it is from latin, I set it into italics. Thus, whatever I did, the text was cut of there, and the online tool was anyhow going further and created more html's, beginning with the new heading.
When I tried to edit the respective text that was cut of in Notepad++, I noticed that the italics tags were - strange enough - removed. I saved it to html, inserted it to the epub, and lo! it could be saved in Sigil.
As soon as I tried to tag the Expression in italics, the same behaviour as before occured again.
Enigmatic, because other words in the text are set into italics, too, and with no problem occuring.

Edit: No, obviously Sigil does not accept any word set into italics.